Whereas these valves used to last about 10 years in service, the most recent two in my car lasted 14 months and 10 months respectively. Also in the past few years I have replaced a few of these parts for other people, the comment always being that the failed part was fairly new (and they looked fairly new as well). I don't have exact data on other folk's parts, but it is obvious these valves have been bad for at least a few years now.

The failure point is not a crack or clean tear. The rubber has the consistency of sticky chewing gum, a result of less than 10 months soaking in standerd green antifreeze, 50/50 ethylene-glycol and water (bad rubber).

Service dates and miles in my car from first restoration:
12/15/86 000,250  New heater valve
12/30/98 132,865  Replace heater valve (used from prior engine) (12-yr, 132,000 mi)
8/18/06  202,099  New heater valve (8-yr, 69,232 mi) - That's the used one 8 yrs.
5/7/17   368,450  Replace heater valve (11-yr, 166,000 mi)
7/19/18  409,287  New heater valve. (14-mo, 40,837 mi)
5/1/19   440,490  Replace heater valve (9-1/2-mo, 31,203 mi)

Recent purchase records I can find:
4/24/17 from Moss Motors, Goleta CA
6/19/17 from Moss Motors, Goleta CA
8/9/18  (2) from Robert Pieces D'Autos Anglises Inc in Laval, QC, Canada (in Moss packages)
4/22/19 from Moss Motors, Goleta CA